Macrocycle formation, typically from M-7 to M-20, in anionic polycaprolactam (PCL) has been thoroughly investigated by running the anionic synthesis of PCL in different experimental conditions. Reaction parameters have been changed, paying special attention to avoid conditions that can favor the formation of side products during the anionic polymerization of epsilon-caprolactam (CL). Namely, all syntheses have been performed in isothermal conditions. Formation and content of macrocyclic PCL have been evaluated as functions of polymerization time as well as activator nature and concentration. Considering that anionic PCL formation is a kinetically controlled process, the present work has been also aimed to investigate the content of both low mass cyclic and macrocyclic species after prolonged thermal treatments (annealing tests) in order to analyze our data in terms of thermodynamic equilibrium. High molar mass cyclic oligomers (M-7-M-20) have been monitored by size exclusion chromatography (SEC) and MALDI mass spectrometric analysis, while high performance liquid chromatography (HPLC) has been helpful for a quantitative evaluation of lower molar mass species. On the basis of our results, possible mechanisms of macrocyclic PCL formation have been proposed.